Method of controlling rolling of metal strips

ABSTRACT

IN A METHOD OF ROLLING A METAL STRIP, A PREDETERMINED PATTERN FOR VARYING WITH TIME THE ROLL PERIPHERAL SPEED THAT CAN MAINTAIN THE STRIP TENSION AT A CONSTANT VALUE IS STORED IN A COMPUTER, THE VARIATION IN THE ROLL OPENING IS DETECTED BY SAID PATTERN OR A TENSION METER OR STRIP GAUGE METER ON THE DELIVERY SIDE OF A ROLLING MILL OR A LOAD CELL MOUNTED ON THE ROLLING MILL OR COMBINATIONS THEREOF, AND THE PRELIMINARY CONTROL OF THE VARIATION WITH TIME OF THE ROLL PERIPHERAL SPEED IS EFFECTED BY THE DETECTED VARIATION IN THE ROLL OPENING SO AS TO COMPENSATE FOR THE VARIATION IN THE STRIP GAUGE. IN ROLLING METAL STRIP COMPOSED OF A NUMBER OF WELDED METAL BANDS OF DIFFERENT GAUGES, SAID PATTERN IS PREDETERMINED WITH REGARD TO THE VARIATION OF THE STRIP GAUGE AT THE WELDS, SO AS TO MAINTAIN THE STRIP TENSION ALWAYS A CONSTANT VALUE.

Nov. 9, 1971 Filed May 16, 1969 ROLLING LOAD P (ton) THlCKNESS OF OIL FILM ,u

TOHRU ARIMURA ETAL 3,618,348

METHOD OF CONTROLLING ROLLING OF METAL STRIPS 9 Sheets-$heet 2 3 FIG ELASTICITY CURVE AT PLASTICITY gaze NORMAL ROLLING OPERATION FFICIENT ELASTICITY CURVE AT cm" OF S- neouceo SPEED o smmou s hp-fl ho 7 ROLL OPENING 5 (mm) BEARING OIL FILM UE STRIP GAUGE 3 (mm) T0 SPEED REDUCT ON FIG 4 500 P (M/mln) 400 400- 500 88 7 OVER o i a OIOO25O 500 750 KDO IZSOISOO I750 (TON) ROLLING LO NOV. 9, 1971 TQHRU ARlMURA ETAL 3,618,348

METHOD OF CONTROLLING ROLLING OF METAL STRIPS Filed May 16, 1969 9 Sheets-Sheet w rf) NOLLQIEH :IO lNBIOHdEIOO NOV. 9, 1971 TOHRU ARMURA ET AL 3,618,348

METHOD OF CONTROLLING ROLLING OF METAL STRIPS Filed May 16, 1969 9 Sheetsheet 4 fifhi l o BVION Nov. 9, 1971 TOHRU ARIMURA 3 METHOD OF CONTROLLING ROLLING 0F METAL STRIP Filcd May 16, 1969 9 Sheets-Sheet 5 NOV. 9, 1971 TOHRU M R ETAL 3,618,348

METHOD OF CONTROLLING ROLLING OF METAL STRIPS Filed May 16, 1969 9 Sheets-"Sheet 6 F l G. 9

NQ3 N94 N95 1r 77 AW METHOD OF CONTROLLING ROLLING OF METAL STRIPS.

Filed Ma 16, 1969 1971 TOHRU ARIMURA ETAL 9 Sheets-Sheet 7 NOV. 9 197] TQHRU ARlMURA ETAL 3,618,348

METHOD OF CONTROLLING ROLLING 0F METAL STRIPS Filed May 16, 1969 9 Sheets-Sheet E;

Nov. 9, 1971 TOHRU ARIMURA ETA!- 3,618,348

METHOD OF CONTROLLING ROLLING OF METAL STRIPS Filed May 16, 1969 9 Sheets-Sheet 9 F I (5. l2

NO. 43 No.446 NO.5;49

United States Patent Office US. Cl. 72-7 14 Claims ABSTRACT OF THE DISCLOSURE In a method of rolling a metal strip, a predetermined pattern for varying with time the roll peripheral speed that can maintain the strip tension at a constant value is stored in a computer, the variation in the roll opening is detected by said pattern or a tension meter or strip gauge meter on the delivery side of a rolling mill or a load cell mounted on the rolling mill or combinations thereof, and the preliminary control of the variation with time of the roll peripheral speed is effected by the detected variation in the roll opening so as to compensate for the variation in the strip gauge. In rolling a metal strip composed of a number of welded metal bands of different gauges, said pattern is predetermined with regard to the variation of the strip gauge at the welds, so as to maintain the strip tension always a constant value.

BACKGROUND OF THE INVENTION This invention relates to a method of controlling rolling of metal strips, and more particularly to a novel method of controlling the rolling speed of a metal strip when it is accelerated or decelerated, such as at the welds between a number of welded metal bands of difi'erent gauges.

According to a modern practice, metal plates or sheets of the required gauge are mass produced by hot and cold continuous rolling operations in order to reduce the cost of the products. Under these circumstances, a metal band of a predetermined size is hot rolled from a slab and two or three such bands are welded together to form one continuous strip, which is then cold rolled to obtain a product of a given gauge. It is known that such process steps are most suitable for the mass production of products of the same size. However, rolling of the products of the same size still involves various problems. For example, the strip is accelerated at the commencement of the rolling operation and decelerated near the end thereof, and as is well recognized in the art such acceleration and deceleration results in a variation in the gauge of the resultant products. Where a strip formed by welding together two or three coils of hot rolled metal bands of equal size is cold rolled, it is necessary to reduce the normal rolling speed and then increase it again when the welds of the strip pass through a rolling mill. Such speed control is necessary to avoid breakage of the strip. This problem is more serious where metal bands welded together have different gauges.

Such variations of the gauge due to acceleration and deceleration of the strip is inherent to continuous rolling mills for mass producing products at high efliciencies. To eliminate this problem of gauge variation and so as to maximize the productivity of the rolling mill, it is highly desirable to develop a method of controlling the mill that can prevent variation in the strip gauge at the time of acceleration and deceleration of the rolling mill. Unfortu- 3,618,348 Patented Nov. 9, 1971 nately, none of the prior control systems for rolling mills has succeeded in eliminating this problem.

According to a typical prior art automatic gauge control system as shown in FIG. 1 of the accompanying drawing, a metal strip is payed out from an uncoiler 1, successively rolled by a series of rolling mill stands 2, 3, 4, 5 and 6 and finally taken up by a tension reel 7. During the normal rolling operation of such tandem cold rolling mills, it is well known that the roll opening (roll gap) and the peripheral speed of the working rolls of each mill stand are related to each other according to the rule of constant flow quantity which is based on a relation that the product of the strip gauge rolled by each mill stand and the rolling speed is constant. Thus, the gauge of the strip rolled by each mill stand and the rolling speed thereof are determined according to this rule. A mechanism for establishing this relation includes screw down devices 14, 15, 16, 17 and 18 for respective mill stands which are operated by a control system for controlling the speed variation of a DC motor known in the art as Ward-Leonard systems and comprising motor generator sets 22-21, 26-25, 29-28, 32-31 and 35-34 which are controlled by speed regulators 20, 24, 27, 30 and 33, respectively to control the speed of the screw down device. The rolling speed is controlled by varying the rotational speed of the working rolls of respective mill stands. For example, the rolls of respective mill stands are driven by Ward-Leonard systems comprised by motor generator sets 37-38, 40-41, 43-44, 46-47 and 49-59 controlled by speed regulators 56, 55, 54, 53 and 51 respectively, in the same manner as the screw down devices. The speeds of motors 37, 40, 43, 46 and 49 are detected by tachometers 36, 39, 42, and 48, respectively, and are fed back to respective speed regulators in the usual manner.

In the control mechanism described above, in order to ensure precise finishing of the ultimate gauge, an automatic gauge control device or an AGC system is often incorporated. Usually, two such systems are employed one of which includes an X-ray gauge meter 8 which measures the gauge of the strip on the delivery side of the first stand so as to vary the roll gap in accordance with a deviation Ah, of the strip gauge from a prescribed value. More particularly, a signal representing the speed of the screw down driving motor 22 which is detected by the tachometer 23 and a signal produced by the gauge meter 8 are supplied to the screw down automatic control device 19 to provide the most suitable screw down. This provides the coarse control required to finish the strip to the desired gauge. The AGC system provided for precise finishing on the delivery side of the final mill stand operates to detect a deviation A11 of the gauge of the finished product from a prescribed value by means of an X-ray gauge meter 9 to supply the detected error signal to the speed control device 51 via an automatic tension regulator 52. The generator 50 is also controlled by the output of the tachometer 48 associated with motor 49. In this manner, the tension of the strip between the fourth and fifth mill stands is controlled to reduce said deviation Ah, to zero, thus assuring accurate gauge of the finished strip. In order to stabilize the operation of the AGC system and to assure correct operation of the entire rolling mill installation, it is necessary to provide additional means such as tension meters 10, 11, 12 and 13. These tension meters function to maintain the tension of the strip between adjacent mill stands at proper values according to the deformation resistance of the strip and the rolling force. Excessive tension causes breakage of the strip whereas too low a tension results in slack or a loop in the strip causing two or more folds of the strip to be rolled. This applies an impact load to the rolls, thus damaging them. Even if such folds can pass through the rolling mill without causing any damage thereto, it is extremely difficult to obtain products of uniform gauge. Such problems are caused by the characteristics of the prior system wherein the accuracy of the strip gauge is improved by correcting a small deviation of the strip gauge from a predetermined value in a predetermined pass schedule for a given strip. Especially, with said AGC system in which the deviation of the strip gauge on the delivery side is detected and the detected error is fed back to the speed regulator of the rolling mill, it is impossible to prevent variation in the strip gauge during acceleration and deceleration of the strip.

SUMMARY OF THE INVENTION According to the present invention, the roll gap is adjusted at the time of acceleration and deceleration of the rolling mill, so as maintain the strip tension between adjacent mill stands at the constant value which is maintained during normal rolling operation. To accomplish this, it is necessary to predetermine the pattern of variation of the peripheral speed of the rolls of each mill stand as well as the operating pattern of the screw down corresponding to said speed pattern. Then variations in the strip tension between adjacent mill stands during acceleration and deceleration can be instantly compensated for, so that the rolling mills can enter into stable normal operation. According to this invention, the strip can be accelerated and decelerated without troubles even when producing many types of products in small quantities or when a strip composed of metal bands of different sizes is rolled. Of course, the invention is more advantageous when rolling a continuous strip consisting of welded metal bands of the same size.

It is, therefore, an object of this invention to prevent variation in the strip gauge caused by the increased and decreased rolling speed of the strip, especially at the welded portions thereof whether the rolling is hot or cold rolling. In other words, it is intended to eliminate variations in the gauge of the strip caused by the acceleration at the commencement and deceleration at the end of the rolling operation of a single continuous strip of material, or caused by deceleration and acceleration at the time when welded portions of a strip formed by welding together two or more metal bands of the same size or formed by welding together metal bands of different gauges, are passed through the rolls, thus providing products of high dimensional accuracies.

Another object of this invention is to fully utilize the merits of conventional automatic strip gauge control systems without imparing the accuracy of the gauge during a normal rolling operation.

According to one embodiment of this invention, there is provided a method of controlling the rolling operation characterized by the steps of predetermining a pattern for varying with time the roll peripheral speed that can maintain the strip tension at a predetermined constant value, storing the pattern in a computer, detecting the variation in the roll opening (or gap) by at least one member selected from the group consisting of a program for maintaining said constant tension, a tension meter or a strip gauge meter installed on the delivery side of the rolling mill and a load cell mounted on the rolling mill, or combinations thereof, and effecting preliminary control of the variation with time of the roll peripheral speed, by means of the detected variation in the roll opening so as to compensate for the variation in the strip gauge.

According to a modified embodiment of this invention, when cold rolling metal strips consisting of welded together strips of different gauges, there is provided a method of controlling the rolling operation characterized by the steps of predetermining a pattern of the variation with time of the roll peripheral speed with regard to the variation of the strip gauge at the welds, storing said pattern in a computer, detecting the variation in the roll opening by at least one member selected from the group consisting of a program regarding variation in the strip gauge at the welds, a tension gauge or a strip gauge meter installed on the delivery side of the rolling mill and a load cell mounted on the rolling mill, or combinations thereof, and effecting preliminary control of the variation with time of the roll peripheral speed by means of the detected variation in the roll opening so as to maintain the strip tension always at a constant value.

Further objects and advantages of this invention can be more fully understood from the following detailed description taken in conjunction with the accompanying drawings in which:

FIG. 1 is a diagram of a prior art control system for tandem cold rolling mills;

FIG. 2 is a graph illustrating variations in the strip gauge on the delivery side and in the tension on the entry side of respective mill stands at welded portions of a continuously rolled metal strip;

FIG. 3 is a graph illustrating the generation of gauge variations in a cold rolling mill at times when it is accelerated and decelerated;

FIG. 4 shows variation patterns of the thickness of oil films of the bearing of the back-up roll of a cold rolling mill;

FIG. 5 shows a variation pattern of the coefiicient of friction caused by the rolling speed of a cold rolling mill;

FIG. 6 shows rolling speed control patterns of all mill stands when the strip gauge varies;

FIG. 7 shows rolling speed compensating patterns for welded portions;

FIG. 8 shows a schematic diagram of a program control system utilized to carry out the method of this invention;

FIG. 9 shows a schematic diagram of the entire control system controlled by tension meters provided between adjacent mill stands;

FIG. 10 shows a modified embodiment of the control system utilizing gauge meters provided between adjacent mill stands;

FIG. 11 shows another example of the control system employing load cells mounted on respective mill stands; and

FIG. 12 is a diagram of a combination control system.

DESCRIPTION OF THE PREFERRED EMBODIMENTS As above described, where a metal strip formed by welding together a number of hot rolled metal bands of different gauges is cold rolled continuously, at the welded portions the strip gauge on the delivery side and the strip tension on the entry side of respective mill stands vary greatly so that it is impossible to prevent off-gauge conditions with a conventional speed control system. FIG. 2 shows variations with time of the strip gauge on the delivery side and of the strip tension on the entry side of each mill stand. In FIG. 2 curves TF1 through TF4 shown by dotted lines represent variation in tension while solid line curves DI-Il through DH4 represent variations in the strip gauge. As can be noted from these curves, the variation in the tension on the entry side greatly varies at points where the strip gauge on the delivery side varies. This means that stable rolling operation cannot be provided unless some effective automatic control system is developed.

One of the features of this invention is to provide a method of maintaining the inter-stand strip tension at a constant value. As a result of our experiments, we have found that, if it is possible to absorb variations in the peripheral speed of the roll caused by the accelera tion or deceleration of a rolling mill by varying the extent of the movement of the screw, the inter-stand tension could readily be maintained at a constant value. However, the variation in the rolling speed at the time of acceleration and deceleration is very large. For example, a normal operating speed of the order of 1500 m/min. is usually reduced to about 500 m./min. As shown in FIG. 2, variation in the strip gauge with such speed variation is extremely large. The principal curves of such severe variation can be explained by using curves shown in FIG. 3 which were disclosed by R. B. Sims (Journal of Iron and Steel Institute, 1952, Vol. 172, pp. 2-5) wherein the ordinate represents the rolling load P in ton and the abscissa the roll opening S and the gauge h of the rolled strip, both in mm. According to R. B. Sims, Where the gauge of a blank h is given, the rolling load P required for rolling the blank to a finished gauge h can be shown by a plasticity curve X. Since a rolling mill can be considered as an elastic body having a coefficient of elasticity M, the variation in the roll opening caused by the rolling load can be shown by an elasticity curve Y, provided that the origin of the abscissa represents a zero roll opening under no load condition. As the gauge of the product is substantially equal to the roll opening, the abscissa 11 corresponding to the cross point Z between curves X and Y represents the gauge of the product or the roll opening While the ordinate P represents the rolling load at that time.

Under these conditions, if it is assumed now that the rolling speed is reduced, the plasticity curve X will be shiftedto X due to an increase in the coeflicient of friction ,u., whereas the roll opening will be increased due to a decrease in the thickness of the bearing oil film, thus shifting the elasticity curve Y toward the right to the position Y. As a result, cross-point Z between these curves shifts to a point Z'. Thus, the strip gauge I1 moves to k along the abscissa and the rolling load P to P along the ordinate. The above description explains the reason for the generation of an ofi-gauge condition at the time of acceleration and deceleration. The relationship among variations of the strip gauge, the roll opening and the rolling load shown in FIG. 3 is given by W. C. F. Hessenberg in a treatise entitled. Principles of Continuous Gauge Control in Sheet and Strip Rolling, appearing in Proceedings of Inspection Mechanical Engineers, 1952, vol. 166, pp. 75-81, by the following equation:

AS=AP/M However, as shown by the curves of FIG. 4, since the roll gap or opening varies greatly with the rolling load and the thickness of the bearing oil film when the rolling speed is taken as the parameter, variation in the roll opening must be represented by the following equation:

AS=AS1+AS2 where AS represents the variation in the screw down commanded and A8 the variation in the roll opening due to variation in the thickness of the bearing oil film.

By substituting Equation 3 into Equation 2, the following equation holds for ith stand of the rolling mill.

Thus, as is clear from FIG. 3, where the rolling speed is varied, the aforementioned variation in the coeflicient of friction results in the variation of the rolling load by AP, as well as in the variation of the roll opening by A caused by the variation in the thickness of the bearing oil film. For this reason, it will be noted that variation or deviation in the strip gauge Ah, can be reduced to zero when the roll position is varied by AS, by the adjustment of the screw down.

Assuming now that AV, represents the variation in the rolling speed of a respective mill stand, then from the relation shown in FIG. 3, the following equations hold:

AP =Af (AV (5) AS -=Ag (AP Av Accordingly, the extent of movement of the screw down A5 to be commanded can be derived as follows from Equation 4:

AP, As,, -(As,,+- (7) By taking the rolling speed as the parameter, Equation 8 can be derived from Equations 5 and 6:

Thus, when the relation between the increment AV, in the rolling speed at the time of varying thereof and the increment AS- is incorporated into an on-line computer, it is possible to readily and quickly control the amount of screw down in response to the variation in the peripheral speed of the mill roll.

The increment of the rolling speed or the peripheral speed of the mill roll expressed by Equation I8 must be carefully investigated before commencing the actual rolling operation. Among various factors, the relation between the extent of movement of the screw, or the roll opening and the increment of the peripheral speed of the roll is most important. This relation depends upon such factors as the construction of the rolling mill employed, the type of the lubricant used and the property of the material to be rolled. In addition to this relation, the gauge, tension and speed of the strip should also be taken into consideration in determining the variation pattern wherein the peripheral speed of the roll is taken as the parameter. By incorporating this pattern into the on-line computer as a program, stable rolling operation becomes possible during acceleration and deceleration of the rolling mill. With regard to the quick variation with time of the roll opening in response to the variation in the peripheral speed of the mill roll, as a resut of exhaustive experiments we have obtained following equation:

1': mill stand number A: a symbol denoting an increment S: the LaPlacean h Strip gauge (mm.) on the delivery side of the ith mill stand T The time (in seconds) required for the strip to move from the ith mill stand to the (i+l)th mill stand V The peripheral speed of the roll (m./ sec.)

A B C and D Constant determined by the strip gauges on the entry and delivery sides, tensions on the entry and delivery sides of the ith and (i+l)th mill stands, respectively, property of the material to be rolled, construction of the rolling mill, rolling speed and the like.

From Equation 9, it can be noted that in changing from a certain normal rolling condition to another, in order to maintain the inter-stand tension always at a constant value, it is necessary to preadjust the peripheral speed of the roll of a given mill stand in accordance with the variation in the change of the roll peripheral speed and the variation in the strip gauge on the delivery side of a precedingstand, and the variation in the strip gauge on the delivery side of said given mill stand. Particularly, the second term of Equation 9 shows that as the variation in the strip gauge on the delivery side of the ith mill stand reaches the (i+l)th stand after T seconds, the speed of the (i+1)th mill stand should be precontrolled by taking this effect into consideration. FIG. 6 shows a plot of variation patterns of the roll peripheral speed calculated according to Equation 9, which are also utilized as the screw down patterns for all mill stands.

While these variation patterns constitute fundamental patterns where the strip gauge varies the acceleration and deceleration controls of this invention at the time of commencement and termination of the rolling operation, and the variable speed control for welded portions of a strip formed by welding, a strip section of the same gauge can be operated on by using a simplified form of Equation 9. More particularly, as has been already pointed out, during the acceleration and deceleration operations no gauge variation is permissible. To satisfy this requirement, the compensating pattern should be such that in Equation 1 Ah=0. Thus, all the first, second and third terms in Equation 9 disappear resulting in the following simplified equation:

AV1+ =DyAV However, as shown in FIG. 1, which is the plot of the result of our experiments when the rolling speed is reduced to as low as 500 m./min., the variation in the coefficient of friction becomes significant resulting in a substantial variation in the strip gauge as shown in FIG. 2. As a result, it becomes necessary to again correct the roll peripheral speed so as to compensate for the variation in the coefficient of friction. When corrected in this manner, Equation 10 is changed as follows:

This equation shows that the coefficient of the ith mill stand has an influence upon the percentages of forwarding and backwarding of the (i+l)th mill stand. By this correction, it is possible to maintain constant the inter-stand tension without any variation so that variation in the strip gauge can be precluded. As is evident from FIG. 5, since the increment A of the coefficient of friction is a function of the increment of the rolling speed, it is necessary to establish the variation pattern of the roll peripheral speed expressed by Equation 8 while satisfying Equation 11. This means that the movement of the screw down should be corrected in response to the variation in the roll peripheral speed. In this manner, any variation in the strip gauge can be avoided.

Different from such variable speed control for providing the condition Alq=0, for the variable speed rolling of welded portions of a continuous strip including a plurality of hot rolled metal bands of different gauges, the above mentioned Equation 9 can be applied without any modification. For such a rolling operation the variation in the strip gauge caused by the increment A5 of the roll gap due to variation in the thickness of the bearing oil film and the increment AIL of the coetficient of friction are both represented as the increment All in Equation 9 which is maintained at a predetermined value. FIG. 7 shows speed compensating patterns to be utilized when welded portions are passed through a continuous cold rolling mill. With these patterns, strip gauge can be changed stably when rolling welded portions at different speeds.

FIG. 8 shows a block diagram of a control system utilized to carry out this invention. According to this invention, acceleration, deceleration and strip gauge change commander 76, an on-line computer 75 and a signal converter 77 are added to the conventional automatic strip gauge control system shown in FIG. 1. Responsive to acceleration and deceleration command signals, the on-line computer immediately computes the required quantities of variations of roll opening and roll peripheral speed for each mill stand according to the patterns stored therein. These computed values are converted into analogue quantities by the signal converter 77, the quantity regarding the variation of roll opening being supplied to roll opening adjusting means 19, 24, 27, 30 and 33 and the quantity regarding the variation of the roll peripheral speed to roll peripheral speed adjusting mechanisms 56, 55, 54, 53 and 51 respectively to effect a preliminary adjustment of acceleration or deceleration. In this case, such preliminary adjustments are performed for successive mill stands by due consideration of operating times of these mechanisms and the running speed of the strip. Thus, it is possible to maintain the inter-stand tension of the strip always at a constant value without the risk of breakage and damage of rolls, thus assuring smooth rolling operation, as mentioned hereinabove, such a program control system can be applied with equally satisfactory results not only for the variable speed rolling of strips of constant gauge, but also to the accelerated and decelerated rolling of strips comprising welded portions of different gauges. To accomplish this, it is only necessary to incorporate the program expressed by Equation 9 into the computer.

To carry out the novel method, other systems may be substituted for the program control system described above. FIG. 9 shows one of such alternative systems in which unique utilization is made of interstand tension meters 10, 11, 12 and 13. When compared with the program control system shown in FIG. 8, although the response characteristic of the control system utilizing tension meters is not so good, the latter system is applicable to rolling mills having relatively low rolling speed. In this case, it should be noted that variations in inter-stand tensions detected by various tension meters are not independent from each other. Thus, such variations are effected by roll peripheral speeds before and after a given tension meter, strip gauge on the delivery side and the tension of the preceding mill stand. For this reason, to calculate required variations of the roll opening and roll peripheral speed, a term representing the increment of the tension should be added to Equation 9. This relation can be expressed by the following experimental equation:

Where A,, B,, C' E, and F' are constants as determined by various rolling conditions described in connection with Equation 9, A l, represents the increment of the screw down of the ith mill stand, AV, the increment of the roll peripheral speed and Ari, the increment of tension at that time.

When rolling welds between different gauge strip portions at accelerated or decelerated speeds, in order to maintain the inter-stand tension at a constant value it is necessary to make zero the variation of each inter-stand tension in Equation 12, thus The change in the setting of the roll peripheral speed of each mill stand required to satisfy this condition is given by the following equation:

Thus, instead of the pattern expressed by Equation 9 regarding FIG. 9, a pattern expressed by Equation 13 is incorporated into the on-line computer 75. In this case, a screw down change command is given to each stand, since the variations in tension at respective preceding mill stands are measured by respective tension meters 10, 11, 12 and 13 and applied to the converter 77, while the roll peripheral speeds are being measured by tachometers 36, 39, 42, 45 and 48 and are then supplied to the same converter. Signals supplied to the converter 77 are 0011- verted into digital quantities and are then supplied to the computer 75 to obtain the desired increment of the roll peripheral speed necessary for a given mill stand. Such increments are applied to roll peripheral speed adjusting 9 mechanisms 56, 55, 54, 53 and 51 to promptly compensate for variations in inter-stand tensions whereby they are maintained constant assuring the stable normal rolling operation.

While the above description refers to rolling at accelerated and decelerated speeds of welds between strips of different gauges, to effect the same rolling operation for a continuous strip of the same (or constant) gauge, Equation 13 must be modified. More particularly as has been discussed in connection with Equation 11, it is necessary to use a compensating pattern wherein the increment Ah of the strip gauge is made to zero. Accordingly, all of the first to third terms in Equation 13 are eliminated thus obtaining following equation;

As the increment of the tension of this equation is a function of the increment V, of the rolling speed, it is important to establish the pattern for varying the roll peripheral speed expressed by Equation 8 concurrent with the satisfaction of Equation 14. By incorporating this program into the on-line computer 75 signals derived from respective inter-stand tension meters can be utilized to make anticipatory control of the roll peripheral speed so as to eliminate gauge variations.

FIG. shows a modified control system wherein a strip gauge meter is installed on the delivery side of each mill stand to effect the variable speed rolling operation. In this embodiment, X-ray gauge meters 8, 9, 57, 58 and 59 identical to those shown in FIG. 1 are installed on the delivery sides of respective mill stands. With these gauge meters, Equation 9 or 8 can be applied without any modification for the variable speed rolling control of welds between strips of different gauges as well as a continuous strip of the same gauge. In computing according to the pattern expressed by Equation 9 or 8, signals measured by strip gauge meters 8, 57, 58, 59 and 9 and tachometers 36, 39, 42, 45 and 48 are applied to the computer 75 via converter 77 and the computed values are fed to respective mill stands.

FIG. 11 shows a still further modification of the control system wherein conventional load cells are substituted for strip gauge meters shown in FIG. 10. Thus, load cells 60, 61, 62, 63 and 64 are mounted on respective mill stands and the detected rolling forces are applied to the computer 75 through the converter 77 together with signals detected by tachometers associated with respective rolls. The relationship among the rolling force, strip gauge and rolling speed of this control system is identical to that already discussed in connection with FIG. 3. Accordingly, various equations described withreference to FIG. 8 are applicable to this control system without any modification. More particularly, Equation 8 for the variable speed rolling of a continuous strip of the same gauge and Equation 9 for the variable speed rolling of welds between strips of different gauges are stored in the on-line computer 75 to perform computing based on the measured values of respective load cells. Control values thus obtained are successively applied to next succeeding mill stands. The modified control system employing load cells can correctly determine without any time lag variations in strip gauge on the delivery sides of respective stands and is cheaper than the employing strip gauge meters.

Thus the control system shown in FIG. 1 is more advantageous than that shown in FIG. 10.

The above described four basic control systems, that is, those utilizing a program, a tension meter, a strip gauge meter and a load cell, respectively, are typical examples of this invention. As discussed hereinabove with reference to respective systems, although they can be used independently, their applications are substantially limited. For example, according to our experiments it was found that the program control system illustrated in FIG. 8 cannot completely eliminate the variation in the interstand strip tension where the strip gauge varies greatly or the rolling rolls become eccentric. Further, the control system illustrated in FIG. 9 and utilizing inter-stand tension meters is advantageous in that it can directly'detect the variation in tension but its response to the screw down change command is low. However, the control systems utilizing a strip gauge meter and a load cell respectively which are illustrated in FIGS. 10 and 12, respectively, can detect large variations in the strip gauge but their response to the screw down change command is slower than that of the control system shown in FIG. 8. As a consequence, a suitable combination of said control systems can greatly improve the control ability by balancing each against other their merits and faults, whereby even a very small variation in the inter-stand strip tension can be sufficiently compensated for.

FIG. 12 shows one example of such a combination control system utilizing inter-stand tension meters 1013 shown in FIG. 9 and load cells 6064 shown in FIG. 11. The control system shown in FIG. 12 is essentially a program control system controlling the roll peripheral speed variation of respective mill stands according to Equation 9. However, as has been pointed out hereinabove, with this control system it is difiicult to compensate for the strip gauge error Ah, other than those caused by the screw down change command so that it is necessary to use the following equation instead of Equation 9.

Ah, in equation can be obtained from following Equation 16 which is a modified form of Equation 1 When a control system shown in FIG. 11 and including load cells is used, the increment or error A'h, of the strip gauge caused by external disturbances and expressed by Equation 16 can be readily determined. More particularly, where the increment AP, deviates from M-AS,, it is necessary to substitute Equation 9 for Equation 16 to perform continuous correction by assuming that the variation in the strip gauge is caused by other factors than the screw down change command. The inter-stand tension meters shown in FIG. 9 can immediately detect whether such two step control can maintain the inter-stand tension at a constant value or not. Utilization of such inter-stand tension meters can detect and compensate for even a very small variation in the tension. Of course, in this case too, the pattern for varying with time the roll peripheral speed of each mill stand should follow Equation 12 regarding FIG. 9, in the combined controlsystem shown in FIG. 12, as Equation 12 contains many terms already analyzed by Equation 9. It is necessary to consider unknown increments alone so that suflicient compensation can be made with following equation.

The pattern for varying with time the roll peripheral speed according to Equation 17 is relatively simple so that it is not necessary to rely upon the on-line computer 75. Since F, and F' are factors that can be experimentally determined from the above described rolling conditions, by providing minor loops containing amplifiers 65, 66, 67, 68 and 69, 70, 7-1 of suitable amplification factors, minute variations in the inter-stand tension can be readily compensated for. Such a minor loop control system with feedback circuits has a quicker response than the control system shown in FIG. 10. FIG. 12 shows one example of combination systems wherein merits and faults of the basic systems balance each other, it being understood that any combination of said four basic systems may be utilized. Selection of a particular combination is determined dependent upon various apparatus associated with the rolling mill and the speed setting.

The above description of the combination control system shown in FIG. 12 refers to the control for the variable speed rolling of welds between strips of different gauges. Control for the variable speed rolling of a continuous strip of uniform gauge will be obvious from equations regarding said four basic control systems, so that description of such a control is believed unnecesary.

While the above description refers to a continuous cold rolling mill, it will be clear that this invention is not limited to the control of the variable speed rolling of such rolling mill, but is equally applicable to a single stand reversible rolling mill. Although the single stand rolling mill essentially comprises uncoiler 1, first stand 2 and take up reel 7 shown in FIG. 1, generation of off-gauge conditions at the time of variable speed drive is quite similar to the above described continuous mills, so that any one of said four basic control systems or combinations thereof may be used with equal results.

With the novel anticipatory control method of the present invention, not only the rolling operation under ordinary reduced or increased speed at the time of commencement or termination thereof, but also the variable speed rolling operation for welds can be made very stable. Further, such variable speed rolling operations can be transferred to the normal rolling operation without any accompanying variation in the inter-stand tension regardless of the gauge of the hot rolled metal bands welded together. This eliminates various process problems throughout a mill factory, including handling of ingots, slabs, hot rolled metal bands and cold rolled strip coils as well as troubles and non-smoothness of planning of rolling many types of strips in small quantities. Moreover, elimination of adjusting time at the time of changing the size of the products and reduction of undesired gauge variations in the resulting products greatly increases productivity.

What is claimed is:

1. In a multi-stand apparatus for rolling a metal strip between pairs of rolls, a method of maintaining the interstand tension constant at the time of a predetermined change in roll peripheral speed by predicting and automatically further controlling the roll peripheral speed at each stand during a rolling operation, comprising:

pre-determing a basic pattern for further altering roll peripheral speed at each stand in addition to said predetermined change in roll peripheral speed to maintain the strip tension at a pre-determined constant value during said predetermined speed change, using the roll opening as a parameter;

detecting a change in the roll opening;

computing values corresponding to a required amount of said further alteration of roll peripheral speed at a plurality of said stands as a function of said detected change in roll opening and as a function of said predetermined basic pattern; and

feeding forward said computed values to a plurality of said stands to further alter the roll peripheral speeds at said stands, in addition to said predetermined change in roll peripheral speed, to compensate for the variation in strip gauge and to maintain the interstand tension constant.

2. A method as set forth in claim 1 wherein said change in the roll opening is detected by a fixed program.

3. A method as set forth in claim 1 wherein said change in the roll opening is detected by measuring inter-stand tension with an inter-stand tension meter.

4. A method as set forth in claim 1 wherein said change in roll opening is detected by measuring the thickness of the strip with a thickness meter at the delivery side of a rolling stand.

5. A method as set forth in claim 1 wherein said change in roll opening is detected by measuring the rolling force at a stand with a rolling force meter.

6. A method as set forth in claim 1 wherein said change in roll opening is detected by the combination of a predetermined fixed program, measuring inter-stand tension, and measuring rolling force at a stand.

7. A method as set forth in claim 1 wherein said change in roll opening is detected by the combination of a predetermined fixed program, measuring inter-stand tension, and measuring the thickness of the strip at the delivery side of a stand.

8. In a multi-stand rolling apparatus for rolling a metal strip between pairs of rolls, said metal strip comprising strip segments of different gauges welded together, a method of maintaining the inter-stand tension constant at the time of a predetermined change in roll peripheral speed by predicting and automatically further controlling the roll peripheral speed at each stand during a rolling operation comprising:

pre-determining a basic pattern for changing the roll peripheral speed at each stand as a pre-determined function of variation of strip gauge at the welds and for further altering roll peripheral speed at each stand in addition to said pre-determined change in roll peripheral speed to maintain the strip tension at a pre-determined constant value during said predetermined speed change, using the roll opening as a parameter;

detecting a change in the roll opening;

computing values corresponding to a required amount of said further alteration of roll peripheral speed at a plurality of said stands as a function of said detected change in roll opening and as a function of said pre-determined basic pattern; and

feeding forward said computed values to a plurality of said stands to further alter the roll peripheral speeds at said stands, in addition to said pre-determined change in roll peripheral speed, to compensate for the variation in strip gauge and to maintain the inter-stand tension constant.

9. A method as set forth in claim 8 wherein said change in roll opening is detected by a fixed program.

10. A method as set forth in claim 8 wherein said change in the roll opening is detected by measuring interstand tension with an inter-stand tension meter.

11. A method as set forth in claim '8 wherein said change in roll opening is detected by measuring the thickness of the strip with a thickness meter at the delivery side of a rolling stand.

12. A method as set forth in claim 8 wherein said change in roll opening is detected by measuring the rolling force at a stand with a rolling force meter.

13. A method as set forth in claim 8 wherein said change in roll opening is detected by the combination of a pre-determined fixed program, measuring inter-stand tension, and measuring rolling force at a stand.

14. A method as set forth in claim 8 wherein said change in roll opening is detected by the combination of a pre-determined fixed program, measuring inter-stand tension, and measuring the thickness of the strip at the delivery side of a stand.

References Cited UNITED STATES PATENTS 3,169,421 2/1965 Bloodworth 7211 3,186,201 6/1965 Ludbrook et al 729 3,332,263 7/1967 Beadle et al 727 3,365,920 1/1968 Maekawa et al. 7210 MILTON S. MEHR, Primary Examiner 

